Positron emission tomography radiotracer for diseases associated with translocator protein overexpression, translocator protein-targeting ligand for fluorescence imaging-guided surgery and photodynamic therapy, and production methods therefor

ABSTRACT

Disclosed is a method for producing a fluorine-18-labeled, translocator protein overexpression-targeting PET radiotracer including: preparing a fluorine-18 reaction solution; producing a ((4-(6,8-dichloro-3-(2-(dipropylamino)-2-oxoethyl)imidazo[1,2-a]pyridin-2-yl)phenyl) (aryl)iodonium) anion precursor as an iodonium salt precursor; producing a 2-(6,8-dichloro-2-(4-(4,4,5,6-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)imidazo[1,2-a]pyridin-3-yl-N,N-dipropylacetamide precursor as a boron ester precursor; producing an iodonium salt precursor reaction solution; preparing a boron ester precursor reaction solution; and producing a radiotracer composition containing a fluorine-18-labeled radiotracer by reacting the iodonium salt or boron ester precursor reaction solution with the fluorine-18-labeled reaction solution. The use of the fluorine-18-labeled PET radiotracer makes it possible to diagnose patients with various brain diseases and tumors by obtaining new images of neuroinflammation, stroke and tumors associated with translocator protein overexpression through PET. In addition, by virtue of the long half-life of fluorine 18, the PET radiotracer can provide brain neuroinflammation and tumor imaging diagnostics to a larger number of patients compared to conventional carbon-11 tracers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of PCT/KR2018/015117 filed on Nov.30, 2018, which claims priority to Korean Application No.10-2018-0007226 filed on Jan. 19, 2018, which applications areincorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a fluorine-18-labeled positron emissiontomography radiotracer for targeting translocator proteinoverexpression, a fluorescent ligand for fluorescence imaging-guidedsurgery and photodynamic therapy, and production methods therefor, andmore particularly to: a fluorine-18-labeled positron emission tomography(PET) radiotracer which is produced using an iodonium salt or boronester precursor and has an excellent ability to be selectively andspecifically taken up into inflammatory regions in neuroinflammation andtumor models; a translocator protein overexpression-targetingfluorescent ligand for fluorescence imaging-guided surgery andphotodynamic therapy, which is produced by introducing a fluorescentmaterial instead of fluorine-18; and production methods therefor.

BACKGROUND ART

Microglial cells of the central nervous system contribute to theactivation and homeostatic maintenance of the nervous system, andfunction to maintain neurons or cause apoptosis by secretingneurotrophins, nitric oxide, proinflammatory cytokines, or the like. Infact, the activation of microglial cells has been reported in variousneurodegenerative diseases such as Alzheimer's disease, Parkinson'sdisease, and Huntington's disease, and diseases such as stroke, braininjury, and brain infection. It is also known that the deposition ofbeta amyloid, which is a factor associated with the onset andprogression of Alzheimer's disease, causes the activation of microglialcells.

So far, it has been reported that the activation of microglial cellsoccurs due to increased expression of the 18-kDa translocator protein(TSPO) present in the mitochondrial membrane, begins within a few hoursand continues for several days after disease onset. Therefore, themeasurement of the expression level of TSPO in microglial cells invarious central nervous system diseases can be used as an in vivobiomarker to assess activation of the cells during the neuroinflammatoryprocess. In practice,(R)-N-methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)isoquinoline-3-carboxamide([¹¹C]PK11195) labeled with C-11 (half-life=20.4 min) as a radiotracerfor positron emission tomography (PET) for assessment of TSPO was firstdeveloped in 1984.

However, the widespread use of [¹¹C] PK11195 has been limited due toproblems with the carbon-11 used, such as a short-half life,non-specific binding in the body's brain, and a low signal-to-noiseratio. Over the past 20 years, there have been developed various newradiotracers which overcome the disadvantages of [¹¹C] PK11195 forneuroinflammatory imaging, and, as one of them, there was developedN-5-fluoro-2-phenoxyphenyl)-N-(2, 5-dimethoxybenzyl) acetamide ([¹¹C]DAA1106) labeled with C-11 which showed a 4-fold higher uptake and whosein vivo metabolites did not pass through the blood brain barrier.However, [¹¹C]DAA1106 has also been reported to have the problem ofshowing low TSPO-specific signals in TSPO. C-11-labeledAT-acetyl-N-(2-methoxybenzyl)-2-phenoxy-5-pyridinamine ([¹¹C] PBR28)developed to overcome the pharmacokinetic disadvantages of this[¹¹C]DAA1106) has a high signal-to-noise ratio while maintaining thebasic chemical structure of [¹¹C]DAA1106, and thus has been verified asan imaging radiotracer for translocator protein overexpression andresearched clinically. However, since [11C] PBR28 is also a compoundlabeled with carbon-11 having a short half-life, it is a radiotracerthat can be used only for a short time after production, and hasdisadvantages in that it involves a high possibility of radiationexposure and can be applied only to a maximum of two patients dependingon the number of PET systems after produced once.

In contrast, another positron-emitting nuclide, fluorine-18, has arelatively long half-life (half-life=109.8 minutes), and a method oflabeling a target compound with fluorine-18 by an organic syntheticmethod is easy and shows a high yield. Accordingly, fluorine-18 can beapplied to diagnosis by radiotracers in a plurality of PET systems overa relatively long period of time after production. Due to theseadvantages, various PET imaging radiotracers labeled with fluorine-18have been developed so far by many research groups. In recent years, thepresent inventors have also developed fluorine-18-labeled2-(2-(4-(2-fluoroethyl)phenyl)-6,8-dichloroimidazo[1,2-a]pyridine-3-yl)-N,Ar-dipropylacetamide([¹⁸F]CB251) as a radiotracer for TSPO PET imaging, which has a high invitro binding affinity and shows high TSPO-binding affinity and a highsignal-noise ratio in preclinical PET imaging studies. The aliphaticethyl fluorine-18 used in the synthesis of [¹⁸F]CB251 has an advantagein that the labeling method is simple, but has disadvantages in that itcan be easily metabolized in vivo and in that when fluorine-18 ionsproduced by the metabolism, which are mainly taken up into bone, aretaken up into bone, an image having a lower signal-to-noise ratiocompared to a target site is provided. To overcome these advantages ofthe aliphatic fluorine-18 label, many research groups have attempted todevelop compounds in which aromatic compounds are labeled directly withfluorine-18 to increase the in vivo stability of fluorine-18. Thisaromatic fluorine-18 labeling method is useful for providing in vivoimages having a high signal-to-noise ratio by strong carbon-fluorine(C(sp2)-F) bonding, but the reactivity of the aromatic nucleophilicfluorine-18 label is lower than that of the aliphatic fluorine-18 label,and hence various precursors and reaction techniques for increasing thereactivity have still been developed. Accordingly, there is a need for aradiotracer capable of targeting overexpression of a disease-specifictranslocator protein while allowing simple and efficient labeling withthe radioisotope fluorine-18 at the aromatic position of the targetcompound.

In recent years, high translocator protein overexpression has been foundeven in cancer cells such as brain cancer, breast cancer, prostatecancer, and bladder cancer cells along with neuroinflammation.Therefore, it is expected that ligands, which have a high bindingaffinity for translocator protein and provide selective in vivo imaging,will expend their applications not only to radiotracers for PET imagingbut also to fluorescent ligands for use in fluorescence imaging-guidedsurgery and photodynamic therapy which can visually provide informationon tumor distribution by providing optimal images during surgery throughthe introduction of fluorescent materials.

SUMMARY OF THE DISCLOSURE

An object of the present invention is to provide: a PET imagingradiotracer for the diagnosis of neuroinflammation and cancer diseasesassociated with translocator protein overexpression, which is producedby labeling a translocator protein-targeting ligand with thepositron-emitting nuclide fluorine-18; a fluorescent ligand forfluorescence imaging-guided surgery and photodynamic therapy, which isproduced by introducing a fluorescent material instead of fluorine-18 tothe same ligand; and production methods therefor.

According to one aspect of the present invention, there is provided amethod for producing a fluorine-18-labeled PET imaging radiotracer fortargeting translocator protein overexpression, the method including thesteps of: preparing a fluorine-18 reaction solution by adding, toacetonitrile (CH₃CN), water having dissolved therein fluorine-18produced from a cyclotron, together with a phase transition catalyst,followed by heating to a temperature of 85 to 95° C.; producing eitheran iodonium salt precursor (type A precursor) by reacting2-(4-trimethyltinaryl-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)dipropylacetamidewith a (diacetoxy)iodoarene derivative, or a boron ester precursor (typeB precursor) by reacting2-(4-bromoaryl-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)dipropylacetamidewith bis(pinacolato)diboron; preparing an iodonium salt precursorreaction solution by dissolving the iodonium salt precursor and2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) in CH₃CN; preparing a boronester precursor reaction solution by dissolving the boron esterprecursor and a copper catalyst (copper(II) trifluoromethanesulfonate,or tetrakis(pyridine)copper(II) triflate) in dimethylformamide (DMF);and either producing a radiotracer composition containing afluorine-18-labeled radiotracer compound by adding the iodonium saltprecursor reaction solution to the fluorine-18 reaction solution,followed by heating and reaction, or producing a radiotracer compositioncontaining a fluorine-18-labeled radiotracer compound by adding theboron ester precursor reaction solution to the fluorine-18 reactionsolution, followed by heating and reaction.

The method may further include the first purification step of purifyingthe radiotracer composition by adding an aqueous hydrochloric acidsolution to the radiotracer composition, followed by adsorption onto aC18 Sep-Pak cartridge, washing with water, and then elution withethanol.

The method may further include the second purification step of purifyingthe radiotracer composition using a high-performance liquidchromatography (HPLC) system equipped with a 244 to 264 nm UV detectorand a radioisotope gamma-ray detector.

The step of producing the fluorine-18 reaction solution may be performedby further adding 18-crown-6 ([C₂H₄O]₆)/cesium hydrogen carbonate(CsHCO₃) as the phase transition catalyst to increase the fluorine-18labeling reactivity.

According to another aspect of the present invention, there is provideda ligand and a fluorine-18-labeled PET imaging radiotracer for targetingtranslocator protein overexpression, which are represented by Formula 1below and produced either by a method including the steps of: preparinga solvent-evaporated fluorine-18 reaction solution by adding, to CH₃CN,water having fluorine-18 dissolved therein, followed by heating to atemperature of 85 to 95° C.; producing an iodonium salt precursor (typeA precursor) by reacting2-(4-trimethyltinaryl-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)dipropylacetamidewith (diacetoxy)iodoarene; preparing an iodonium salt precursor reactionsolution by dissolving the iodonium salt precursor and2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) in CH₃CN, and producing aradiotracer composition containing a fluorine-18-labeled radiotracer byadding the iodonium salt precursor reaction solution to the fluorine-18reaction solution, followed by heating and reaction; producing a boronester precursor (type B precursor) by reacting2-(4-bromoaryl-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)dipropylacetamidewith bis(pinacolato)diboron; preparing a boron ester precursor reactionsolution by dissolving the boron ester precursor and a copper catalyst(copper(II) trifluoromethanesulfonate, or tetrakis(pyridine)copper(II)triflate) in dimethylformamide (DMF); and producing a radiotracercomposition containing a fluorine-18-labeled radiotracer by adding theboron ester precursor reaction solution to the fluorine-18 reactionsolution, followed by heating and reaction:

wherein R is ¹⁸F or ¹⁹F, X is C or N, and Y is C or N.

The 2-fluoroaryl-6,8-dichloroimidazopyridine derivative may besynthesized from an iodonium salt or boron ester precursor (type A or Bprecursor) represented by Formula 2 below:

wherein X is C or N, and Y is C or N.

Z in Formula 2 above may be a functional group selected from the groupconsisting of iodobenzene tosylate, iodotoluene tosylate,2-iodo-1,3,5-trimethylbenzene tosylate, 4-iodoanisole tosylate,3-iodoanisole tosylate, 2-iodothiophene tosylate, 3-iodothiophenetosylate, iodobenzene bromide, iodotoluene bromide,2-iodo-1,3,5-trimethylbenzene bromide, 4-iodoanizole bromide,3-iodoanisole bromide, 2-iodocyophene bromide, 3-iodothiophene bromide,iodobenzene iodide, iodotoluene iodide, 2-iodo-1,3,5-trimethylbenzeneiodide, 4-iodoanisole iodide, 3-iodoanisole iodide, 2-iodothiopheneiodide, 3-iodothiophene iodide, iodobenzene triflate, iodotoluenetriflate, 2-iodo-1,3,5-trimethylbenzene triflate, 4-iodoanisoletriflate, 3-iodoanisole triflate, 2-iodothiophene triflate,3-iodothiophene triflate, and pinacol boron ester.

According to still another aspect of the present invention, there isprovided a fluorescent ligand for targeting translocator proteinoverexpression represented by Formula 3 below, which is produced byintroducing a fluorescent dye or a sensitizer, which has a functionalgroup for complementary bonding, to a 2-aryl-6,8-dichloroimidazopyridinederivative precursor substituted with one or more polyethylene glycol(PEG) chains containing a functional group, which is generally(universally) used for bonding with a fluorescent molecule as shown inFormula 4 below:

wherein X is C or N, and Y is C or N. In addition, the number (n) of thepolyethylene glycol chains is 1 to 10. The linker that connects the PEGto the fluorescent dye or the sensitizer may be a compound selected fromthe group consisting of ether, amide, ester, urea, urethane, thiourea,and disulfide, and the PEG is substituted at any one of the 2-, 3- and4-positions of the ring containing X and Y.

The fluorescent ligand having the fluorescent dye or sensitizerintroduced thereto may be synthesized from a2-aryl-6,8-dichloroimidazo[1,2-a]pyridine-3-yl)dipropylacetamideprecursor substituted with one more polyethylene glycol (PEG) chainscontaining a terminal functional group capable of binding with afluorescent molecule as shown in Formula 4 below:

In Formula 4 above, X is C or N, and Y is C or N. In addition, thenumber (n) of the polyethylene glycol chains is 1 to 10. Z in Formula 4may be a functional group selected from the group consisting of acid,alcohol, thiol, amine, isocyanate, isothiocyanate, bromide, iodide,chloride, N-succinimidyl ester, and sulfo-N-succinimidyl ester, and thePEG is substituted at any one of the 2-, 3- and 4-positions of the ringcontaining X and Y.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 depicts a reaction scheme showing a process of synthesizing afluorine-18-labeled radiotracer using an iodonium salt or boron esterprecursor according to one embodiment of the present invention;

FIG. 2 depicts HPLC chromatograms showing the results of separating apure fluorine-18-labeled radiotracer from a synthetic mixture, which isa derivative of Formula 2, in an example of the present invention;

FIG. 3 depicts HPLC chromatograms showing the results of injecting apure fluorine-18-labeled radiotracer, which is a derivative of Formula2, simultaneously with a reference material having a non-radioisotope,in order to check whether the pure fluorine-18-labeled radiotracer isthe same as the reference material, in an example of the presentinvention;

FIG. 4 depicts images showing the uptake of the fluorine-18-labeleldradiotracer [¹⁸F]BS224 ([¹⁸F]BS compound, ¹⁸F-labeled Bari and SeoulNational University compound) in a portion having neuroinflammationinduced by lipopolysaccharide (LPS) injected directly into the rat'sbrain in order to evaluate usefulness against neuroinflammation in amethod for evaluating the biological results obtained using afluorine-18-labeled radiotracer for PET imaging for targetingtranslocator protein overexpression, synthesized according to thepresent invention;

FIG. 5 depicts images showing the uptake of a fluorine-18-labeleldradiotracer in a portion having cerebral ischemia induced in a middlecerebral artery occlusion ((MCA( ) rat model in order to evaluatediagnostic usefulness for stroke and cerebral infarction diseases in amethod for evaluating the biological results obtained using thefluorine-18-labeled radiotracer [¹⁸F]BS224 for PET imaging for targetingtranslocator protein overexpression, synthesized according to thepresent invention;

FIG. 6 is a table comparing the binding affinity of BS224 (Bari andSeoul National University compound 224), which is a ligand for targetingtranslocator protein overexpression according to the present invention,for TSPO or CBR, with the binding affinities of existing compounds knownto bind to TSPO or CBR, in rat cerebrocortical samples;

FIG. 7 compares the PET images acquired using the PET imagingradiotracer [¹⁸F]BS224 for targeting translocator protein overexpressionaccording to the present invention with the PET images acquired usingexisting [¹⁸F]CB251 in normal persons; and

FIG. 8 compares TSPO PET images acquired from a normal person and amidbrain stroke patient using the ligand [¹⁸F]BS224 for targetingtranslocator protein overexpression according to the present invention,and shows a graph comparing the radio activity in the lesion region withthat of the normal person.

DETAILED DESCRIPTION OF THE DISCLOSURE

The terms and words used in the specification and the claims should beinterpreted as having meanings and concepts relevant to the technicalspirit of the present invention based on the principle that an inventormay appropriately define the concept of a term in order to describe hisor her invention in the best way.

The present invention will be described in greater detail below withreference to the accompanying drawings. However, the present inventionis not limited to the embodiments disclosed below, but may beimplemented in various different forms. These embodiments are providedmerely to make the disclosure of the present invention thorough andcomplete and to fully convey the scope of the present invention to thosehaving ordinary skill in the art.

FIG. 1 depicts a reaction scheme showing a process of synthesizing afluorine-18-labeled radiotracer using an iodonium salt or boron esterprecursor according to one embodiment of the present invention.

FIG. 2 depicts HPLC chromatograms showing the results of separating apure fluorine-18-labeled radiotracer from a synthetic mixture, which isa derivative of Formula 2, in an example of the present invention.

FIG. 3 depicts HPLC chromatograms showing the results of injecting apure fluorine-18-labeled radiotracer, which is a derivative of Formula2, simultaneously with a reference material having a non-radioisotope,in order to check whether the pure fluorine-18-labeled radiotracer isthe same as the reference material, in an example of the presentinvention.

FIG. 4 depicts images showing the uptake of the fluorine-18-labeleldradiotracer [¹⁸F]BS224 ([¹⁸F]BS compound, ¹⁸F-labeled Bari and

Seoul National University compound) in a portion havingneuroinflammation induced by lipopolysaccharide (LPS) injected directlyinto the rat's brain in order to evaluate usefulness againstneuroinflammation in a method for evaluating the biological resultsobtained using a fluorine-18-labeled radiotracer for PET imaging fortargeting translocator protein overexpression, synthesized according tothe present invention.

FIG. 5 depicts images showing the uptake of a fluorine-18-labeleldradiotracer in a portion having cerebral ischemia induced in a middlecerebral artery occlusion ((MCA( ) rat model in order to evaluatediagnostic usefulness for stroke and cerebral infarction diseases in amethod for evaluating the biological results obtained using thefluorine-18-labeled radiotracer [¹⁸F]BS224 for PET imaging for targetingtranslocator protein overexpression, synthesized according to thepresent invention.

FIG. 6 is a table comparing the binding affinity of BS224 (Bari andSeoul National University compound 224), which is a ligand for targetingtranslocator protein overexpression according to the present invention,for TSPO or CBR, with the binding affinities of existing compounds knownto bind to TSPO or CBR, in rat cerebrocortical samples.

FIG. 7 compares the PET images acquired using the PET imagingradiotracer [¹⁸F]BS224 for targeting translocator protein overexpressionaccording to the present invention with the PET images acquired usingexisting [¹⁸F]CB251 in normal persons.

FIG. 8 shows TSPO PET images acquired from a normal person and amidbrain stroke patient using the ligand [¹⁸F]BS224 for targetingtranslocator protein overexpression according to the present invention,brain PET images acquired from the same midbrain stroke patient, and agraph comparing the radio activity between the normal brain cell regionand the lesion region in the PET images.

Referring to FIG. 1, a method for producing a fluorine-18-labeled PETimaging radiotracer for targeting translocator protein overexpressionaccording to the present invention may include the steps of:

preparing a fluorine-18 reaction solution by adding, to acetonitrile(CH₃CN), water having dissolved therein fluorine-18 produced from acyclotron, together with a phase transition catalyst, followed byheating to a temperature of 85 to 95° C.;

producing either an iodonium salt precursor (type A precursor) byreacting2-(4-trimethyltinaryl-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)dipropylacetamidewith a (diacetoxy)iodoarene derivative, or a boron ester precursor (typeB precursor) by reacting2-(4-bromoaryl-6,8-dichloroimidazo[1,2-a]pyridin-3-yl)dipropylacetamidewith bis(pinacolato)diboron;

preparing an iodonium salt precursor reaction solution by dissolving theiodonium salt precursor and 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)in CH₃CN;

preparing a boron ester precursor reaction solution by dissolving theboron ester precursor and a copper catalyst (copper(II)trifluoromethanesulfonate, or tetrakis(pyridine)copper(II) triflate) indimethylformamide (DMF); and

either producing a radiotracer composition containing afluorine-18-labeled radiotracer compound by adding the iodonium saltprecursor reaction solution to the fluorine-18 reaction solution,followed by heating and reaction, or producing a radiotracer compositioncontaining a fluorine-18-labeled radiotracer compound by adding theboron ester precursor reaction solution to the fluorine-18 reactionsolution, followed by heating and reaction.

In the present invention, the ligand and the fluorine-18-labeled PETimaging radiotracer for targeting translocator protein overexpressionmay be compounds represented by Formula 1 below:

wherein R is ¹⁸F or ¹⁹F, X is C or N, and Y is C or N.

The 2-fluoroaryl-6,8-dichloroimidazopyridine derivative may besynthesized from an iodonium salt or boron ester precursor (type A or Bprecursor) represented by Formula 2 below:

wherein X is C or N, and Y is C or N.

As shown in Reaction Scheme 1 below, the fluorine-18-labeled radiotracerof Formula 1 may be produced by reacting the iodonium salt or boronester precursor of Formula 2 with a [¹⁸F] cesium fluoride compoundformed through the phase transition catalyst (18-crown-6/cesium hydrogencarbonate) added to increase the fluorine-18 labeling reactivity,thereby labeling the aromatic ring with ¹⁸F:

The reaction product of Reaction Scheme 1 above may be an iodonium saltor boron ester precursor, and Z in Reaction Scheme 1 above may be afunctional group selected from the group consisting of iodobenzenetosylate, iodotoluene tosylate, 2-iodo-1,3,5-trimethylbenzene tosylate,4-iodoanisole tosylate, 3-iodoanisole tosylate, 2-iodothiophenetosylate, 3-iodothiophene tosylate, iodobenzene bromide, iodotoluenebromide, 2-iodo-1,3,5-trimethylbenzene bromide, 4-iodoanizole bromide,3-iodoanisole bromide, 2-iodocyophene bromide, 3-iodothiophene bromide,iodobenzene iodide, iodotoluene iodide, 2-iodo-1,3,5-trimethylbenzeneiodide, 4-iodoanisole iodide, 3-iodoanisole iodide, 2-iodothiopheneiodide, 3-iodothiophene iodide, iodobenzene triflate, iodotoluenetriflate, 2-iodo-1,3,5-trimethylbenzene triflate, 4-iodoanisoletriflate, 3-iodoanisole triflate, 2-iodothiophene triflate,3-iodothiophene triflate, and pinacol boron ester.

In Reaction Scheme 1, Y may be carbon in the case where X is nitrogen,and X may be carbon in the case where Y is nitrogen. Alternatively, bothX and Y may be carbon.

In the production of the fluorine-18-labeled radiotracer compound ofFormula 1, the introduction of ¹⁸F may be performed by a processincluding the steps of: preparing a water-evaporated fluorine-18reaction solution by heating to a temperature of 85 to 95° C. in a CH₃CNsolvent containing 18-crown-6/cesium hydrogen carbonate to [¹⁸F] cesiumfluoride; and transferring the [¹⁸F] cesium fluoride into a reactioncontainer in which the starting material of Formula 2 and TEMPO or acopper catalyst (copper(II) trifluoromethanesulfonate, ortetrakis(pyridine)copper(II) triflate) are dissolved in a CH₃CN or DMFsolvent, followed by heating and reaction.

After the 2-fluoroaryl-6,8-dichloroimidazopyridine derivative havingfluorine-18 introduced thereto is produced by the above-describedprocess, it may be cooled to room temperature and separated/purified byHPLC.

The compound of Formula 2, which is used as a starting material in theproduction of the fluorine-18-labeled radiotracer, has a structure inwhich the benzene or pyridine ring on the right side of the iodoniumsalt precursor is substituted with iodobenzene tosylate, iodotoluenetosylate, or the like, which results in the difference in electrondensity of the benzene or pyridine ring of the iodonium salt precursorbetween the two aromatics on both sides with respect to iodine. As aresult, the substituent exhibits the effect of increasing the yield andselectivity while allowing the right ring of the iodonium salt precursorto be substituted directly with fluorine-18.

The 2-fluoroaryl-6,8-dichloroimidazopyridine derivative represented byFormula 1 may be produced by the method shown in Reaction Scheme 2below:

wherein compound d in Reaction Scheme 2 is a kind of2-fluoroaryl-6,8-dichloroimidazopyridine derivative of Formula 1.

That is, the fluoroaryl-6,8-dichloroimidazopyridine derivative may beproduced through the steps of: producing compound (b) by reactingcompound (a) with dipropylamine in the presence of1,1′-carbonyldiimidazole and TEA in a tetrahydrofuran solvent(derivative production step 1); producing compound (c) by reactingcompound (b) with bromine in a tetrachlorocarbon solvent (derivativeproduction step 2); and producing compound (d) by reacting compound (c)with 2-amino-3,5-dichloropyridine in a dimethylformamide solvent(derivative production step 3).

During the production of the fluoroaryl-6,8-dichloroimidazopyridinederivative, the intermediate product obtained in each step may beseparated/purified by a filtration method, a purification method or thelike known in the organic synthesis field.

The fluorine-18 [¹⁸E]-introduced fluoroaryl-6,8-dichloroimidazopyridinederivative of Formula 1 may be produced from the iodonium salt or boronester precursor represented by Formula 2 below:

wherein Z, X and Y are as defined above with respect to Formula 2.

The iodonium salt or boron ester precursor of Formula 2 may be used as astarting material for producing the fluorine-18 [¹⁸F]-introducedfluoroaryl-6,8-dichloroimidazopyridine derivative of Formula 1. Z inFormula 2 allows the electron density of the right ring of the iodoniumsalt precursor to be different from that of the aromatic compound on theopposite side with respect to iodine, so that it exhibits the effect ofincreasing the yield and selectivity while allowing fluorine-18 to beintroduced directly to the benzene or pyridine on the right side of theiodonium salt precursor.

The produce of introducing a fluorine-containing -Z group to the2-aryl-6,8-dichloroimidazopyridine derivative may be performed by thestep of introducing the -Z group to the right ring of the2-aryl-6,8-dichloroimidazopyridine derivative (compound (e)), as shownin Reaction Schemes 3 and 4 below:

In Reaction Schemes 3 and 4, Z, X and Y are as defined above withrespect to Formula 2.

In Reaction Scheme 3 above, the compounds that are reacted with compound(3) to introduce the -Z group may be (diacetoxy)arene and para-toluenesulfonic acid.

In Reaction Scheme 4 above, the compounds that are reacted with compound(g) to introduce a boron ester group may be bis(pinacolato)diboron and[1,1′-bis(diphenylphosphino)ferrocene] palladium(II) dichloride.

The reaction of Reaction Scheme 3 may be performed by dissolvingdiacetoxyarene in a CH₃CN solvent, adding para-toluene sulfonic acidthereto, and adding thereto dropwise a solution of a composition for theintroduction of the -Z group in a chloroform solvent, followed bystirring at 50° C. for 15 to 18 hours.

The reaction of Reaction Scheme 4 may be performed by adding, to adimethylformamide solvent, a compound for the introduction of the -Rgroup, bis(pinacolato)diboron and [1,1′-bis(diphenylphosphino)ferrocene]palladium(II) dichloride, followed by stirring at 80° C. for 5 hours.

In still another aspect, the present invention also provides asensitizer-labeled, translocator protein overexpression-targeting tracerfor fluorescence imaging-guided surgery and photodynamic therapy, whichis represented by Formula 3 below and produced by a method including thestep of producing a fluorescent ligand through a reaction between a2-aryl-6,8-dichloroimidazopyridine derivative precursor substituted withone or more PEG chains having a functional group, which is generally(universally) used for bonding to a biomolecule as shown in Formula 4below, and a fluorescent dye or photodynamic therapy sensitizer having afunctional group for complementary bonding to the precursor:

wherein X is C or N, and Y is C or N. In addition, the number (n) of thepolyethylene glycol (PEG) chains is 1 to 10. The linker that connectsthe PEG to the fluorescent dye or photodynamic therapy sensitizer may bea compound selected from the group consisting of ether, amide, ester,urea, urethane, thiourea, and disulfide, and the PEG is substituted atany one of the 2-, 3- and 4-positions of the ring containing X and Y.

The PEG chain-substituted 2-aryl-6,8-dichloroimidazopyridine derivativehaving a fluorescent dye or the sensitizer introduced thereto may besynthesized from a PEG chain-substituted2-aryl-6,8-dichloroimidazopyridine derivative precursor represented byFormula 4 below:

In Formula 4 above, X is C or N, and Y is C or N. In addition, thenumber (n) of the polyethylene glycol chains is 1 to 10. Z in Formula 4may be a functional group selected from the group consisting of acid,alcohol, thiol, amine, isocyanate, isothiocyanate, bromide, iodide,chloride, N-succinimidyl ester, and sulfo-N-succinimidyl ester, and thePEG is substituted at any one of the 2-, 3- and 4-positions of the ringcontaining X and Y.

As shown in the Reaction Scheme below, the PEG chain-substituted2-aryl-6,8-dichloroimidazopyridine derivative having a fluorescent dyeor sensitizer introduced thereto as shown in Formula 3 above may beproduced through a reaction between the2-aryl-6,8-dichloroimidazopyridine derivative precursor, substitutedwith one or more PEG chains as represented by Formula 4 above, whichhave a functional group which is generally (universally) used to connecta particular molecule to a biomolecule by covalent bonding, and afluorescent dye or photodynamic therapy sensitizer having a functionalgroup for complementary bonding to the precursor.

The fluorescent dye or sensitizer that is used in the present inventionmay be a compound selected from the group consisting of porphyrin-basedcompounds, porphyrin precursor-based compounds, phthalocyanine-basedcompounds, porphycene-based compounds, chlorine-based compounds,fluorescein-based compounds, anthracene-based compounds, hypericin,furocoumarin-based compounds, chlorophyll derivatives, purpurin-basedcompounds, phenothiazines, methylene blue, violet green, azure C,thionine, nile blue A, hypocrellin, rose bengal, rhodamine 123, IR-700,IR-780, PC-413, and lutetium texaphyrin.

The porphyrin-based compound may be selected from the group consistingof hematoporphyrin derivatives, dihematoporphyrin ether/ester, porfimersodium, tetrasodium-meso-tetraphenylporphyrin-sulphonate, andmetallotetra-azaporphyrin.

The porphyrin precursor-based compound may be selected from the groupconsisting of d-aminolevulinic acid (ALA), d-aminolevulinic acid(ALA)-methyl-, propyl-, and hexyl-esters.

The phthalocyanine-based compound may be selected from the groupconsisting of chloroaluminum tetra-sulfonated phthalocyanine, zinc(II)phthalocyanine, silicone naphthalocyanine, and aluminum sulfonatedphthalocyanine.

The porphycene-based compound may be selected from the group consistingof 9-acetoxy-2,7,12,17-tetra-N-propylporphycene,2-hydroxyethyl-7,12,17-tris(methoxyethyl)porphycene, and23-carboxy-24-methoxycarbonylbenzo(2,3)-7,12,17-tri(methoxyethyl)-porphycene.

The chlorine-based compound may be selected from the group consisting ofmono-aspartyl chlorine e6, diaspartyl chlorine e6, chlorine e6 sodium,and bacteriochlorin.

The fluorescein-based compound may be selected from the group consistingof fluorescein sodium and tetrabromofluorescein-eosin.

The anthracene-based compound may be selected from the group consistingof anthraquinone, acridine orange, and acridine yellow.

The furocoumarin-based compound may be selected from the groupconsisting of 5-methooxypsoralen and 8-methoxypsoralen.

The purpurin-based compound may be selected from the group consisting ofmetallopurpurin and tin etiopurpurin.

The fluorine-18-introduced 2-fluoroaryl-6,8-dichloroimidazopyridinederivative of Formula 1 has a high binding affinity for TSPO present inthe outer mitochondrial membrane in cells, and thus may be a PETradiotracer for diseases related to TSPO overexpression. The positronsreleased from fluorine-18 after binding to TSPO in the body meetelectrons in the body and annihilate. Two gamma-ray energies (511 keV)produced during the annihilation may be collected and a relevant regionshowing high specific expression of TSPO in the body may be directly andnon-invasively imaged by PET.

In addition, the fluorescent molecule-introduced2-aryl-6,8-dichloroimidazopyridine derivative of Formula 3 may bind tocancer cells specifically expressing TSPO in the body by a mechanismsimilar to the above-described mechanism, thus providing an image guideto a correct tumor site during tumor surgery. Alternatively, it may beused as a sensitizer that can more effectively receive the light in thetherapeutic wavelength range from an affected area. In addition, it maybe used in photodynamic therapy in which it binds directly to TSPO in atumor, and then induces cell necrosis when locally exposed to a lightsource. When a fluorescent dye is introduced to the2-aryl-6,8-dichloroimidazopyridine derivative compound having theproperty of binding to TSPO overexpressed in vivo and is injected invivo and the relevant region is irradiated with light having a specificwavelength corresponding to the fluorescent dye after sufficienttargeting of TSPO, the substance bound thereto can be visualized bylight emission, thus providing guidelines for tumor surgery.Alternatively, the 2-aryl-6,8-dichloroimidazopyridine derivative havingintroduced thereto a sensitizer for photodynamic therapy can treat atumor by releasing reactive oxygen species into the relevant region whenreceiving light having a specific wavelength.

Furthermore, when a sensitizer for PDT is introduced to the fluorescentmolecule-introduced 2-aryl-6,8-dichloroimidazopyridine derivative ofFormula 3, there are advantages in that non-invasive therapy is possiblein the same manner as PET, and in that in the case of cancer which canlose the functionality of a normal organ due to surgery, a therapy thatkills only cancer cells by simply irradiating light is possible.

The uptake and release of the 2-aryl-6,8-dichloroimidazopyridinederivative in a brain and a tumor may be controlled by modifying X or Yof the aryl on the right side of the 2-aryl-6,8-dichloroimidazopyridinederivative of Formula 1 or 2 or by changing the position of substitutionof fluorine-18 or a fluorescent dye or sensitizer introduced via one ormore PET chains. If necessary, the uptake and release rates of thederivative may be controlled by increasing the polarity of thesesubstituents.

Therefore, the 2-aryl-6,8-dichloroimidazopyridine derivatives ofFormulas 1 and 3 according to the present invention may advantageouslybe used to determine whether various brain diseases and tumorsassociated with translocator protein overexpression are present or todiagnose and treat the relevant region by administration to mammals,preferably humans.

PREPARATION EXAMPLES Preparation of Starting Material for ProducingIodonium Salt or Boron Ester Precursor Preparation Example 1:Preparation of2-(6,8-dichloro-2-(4-(trimethylstannyl)phenyl)-imidazo[1,2-a]pyridin-3-yl)-N,Nr-dipropylacetamide

(Step 1): Preparation of 4-(4-bromophenyl)-4-oxo-B;N-dipropylbutanamide

3-(4-bromobenzoyl)propionic acid (2.0 g, 7.8 mmol) and1,1′-carbonyldiimidazole (1.4 g, 8.6 mmol) were dissolved intetrahydrofuran (THF, 50 ml) and stirred for 30 minutes. After 30minutes, N,N′-dipropylamine (1.2 mL, 8.6 mmol) and triethylamine (1.3ml, 9.4 mmol) were added thereto, followed by stirring for 3 hours.After the solvent was removed under reduced pressure, a 0.1 N aqueoussolution of hydrogen chloride was added thereto, followed by extractionwith ethyl acetate. The extracted organic layer was dried over sodiumsulfate and filtered, and the remaining organic solvent was evaporatedunder reduced pressure. The residue was purified by columnchromatography to afford the desired compound.

¹H NMR (400 MHz, CDCl₃) δ 0.86 (t, J=7.4 Hz, 3H, CH₃), 0.95 (t, J=7.4Hz, 3H, CH₃), 1.53 (q, J=7.4 Hz, 2H, CH₂), 1.65 (q, J=7.4 Hz, 2H, CH₂),2.77 (t, J=7.4 Hz, 2H, CH₂CO), 3.2-3.4 (m, 6H, CH₂N+CH₂CO), 7.58 (d,J=6.8 Hz, 2H, Ar), 7.86 (d, J=6.8 Hz, 2H, Ar); ¹³C NMR (100 MHz, CDCl₃)δ 198.6, 171.1, 135.8, 132.0, 131.9, 129.8, 129.7, 128.2, 49.7, 47.9,33.9, 27.3, 22.3, 21.1, 11.6, 11.5, 11.4; Anal. Calculated for(C₁₆H₂₂BrNO₂): C, 56.48; H, 6.52; N, 4.12%. Found: C, 56.59; H, 6.50; N,4.11%. MS: calculated for [M]⁺=339 Found: MS m/z (% relative to the basepeak)=339 (5, M+), 239 (base); IR (KBr): 1639, 1687 cm⁻¹.

(Step 2): Preparation of3-bromo-4-(4-bromophenyl)-4-oxo-dipropylbutanamide

To a solution of 4-(4-bromophenyl)-4-oxo-N,N-dipropylbutanamide (2.0 g,5.9 mmol) in tetrachlorocarbon (50 mL), a solution of bromine (0.33 ml,6.5 mmol) in tetrachlorocarbon (1 mL) was added slowly. The reactionmixture was stirred for 3 hours, and then the solvent was removed underreduced pressure. The residue was treated with a saturated aqueoussolution of sodium hydrogen carbonate and extracted with ethyl acetate.The extracted organic layer was dried over sodium sulfate and filtered,and the remaining organic solvent was evaporated under reduced pressure.The residue was purified by column chromatography to afford the desiredcompound.

¹H NMR (400 MHz, CDCl₃) δ 0.83 (t, J=7.4 Hz, 3H, CH₃), 0.98 (t, J=7.4Hz, 3H, CH₃), 1.49 (q, J=7.4 Hz, 2H, CH₂), 1.66 (q, J=7.4 Hz, 2H, CH₂),3.01 (dd, J₁=16.0 Hz, J₂=4.4 Hz, 1H, CH₂CO), 3.1-3.4 (m, 4H, CH₂N), 3.57(dd, J₁=16.0 Hz, J₃=9.9 Hz, 1H, CH₂CO), 5.59 (dd, J₃=9.9 Hz, J₂=4.4 Hz,1H, CHBr), 7.62 (d, J=6.8 Hz, 2H, Ar), 7.91 (d, J=6.8 Hz, 2H, Ar); ¹³CNMR (100 MHz, CDCl₃) δ 192.4, 169.3, 133.2, 132.3, 132.1, 130.7, 130.6,130.5, 128.9, 49.7, 47.6, 40.6, 38.5, 38.4, 22.2, 21.0, 11.5, 11.4;Anal. Calculated for (C₁₆H₂₁Br₂NO₂): C, 45.85; H, 5.05; N, 3.34%. Found:C, 46.03; H, 5.03; N, 3.35%. MS: calculated for [M]⁺=419 Found: MS m/z(% relative to the base peak)=419 (0.5, M+), 319 (base) ; IR (KBr) :1689, 1683 cm⁻¹.

(Step 3): Preparation of2-(2-(4-bromophenyl)-6,8-dichloro-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide

To a solution of 3-bromo-4-(4-bromophenyl)-4-oxo-N,N-dipropylbutanamide(2.0 g, 4.8 mmol) in dimethylformamide (DMF, 25 mL),2-amino-3,5-dichloropyridine (1.0 g, 6.2 mmol) was added. The reactionmixture was stirred under reflux for 18 hours. The solvent was removedunder reduced pressure, and the residue was dissolved in ethyl acetateand treated with a 0.1N aqueous solution of hydrochloric acid, and thenthe organic layer was extracted. The extracted organic layer was driedwith sodium sulfate, and the remaining solvent was removed under reducedpressure. The residue was purified by column chromatography to affordthe desired compound.

¹H NMR (400 MHz, CDCl₃) δ 0.77 (t, J=7.4 Hz, 3H, CH₃), 0.86 (t, J=7.4Hz, 3H, CH₃), 1.2-1.3 (m, 4H, CH₂), 3.12 (t, J=7.7 Hz, 2H, CH₂NCO) ,3.30 (t, J=7.7 Hz, 2H, CH₂NCO) , 4.04 (s, 2H, CH₂CO), 7.30 (d, J=1.9 Hz,1H, Ar), 7.53 (d, J=8.2 Hz, 2H, Ar), 7.60 (d, J=8.2 Hz, 2H, Ar), 8.24(d, J=1.9 Hz, 1H, Ar); ¹³C NMR (100 MHz, CDCl₃) δ 167.4, 144.8, 141.6,133.1, 132.3, 130.9, 125.3, 123.8, 123.1, 122.1, 120.1, 117.9, 50.4,48.5, 30.6, 22.7, 21.4, 11.8, 11.5; Anal. Calculated for(C₂₁H₂₂BrCl₂N₃O): C, 52.20; H, 4.59; N, 8.70%. Found: C, 52.34; H, 4.57;N, 8.74%. MS: calculated for [M]⁺=483 Found: MS m/z (% relative to thebase peak)=483 (3, M+), 128 (base). ESI-MS: calculated for [M−H]⁻=482.Found=482: IR (KBr) : 1698 cm⁻¹.

(Step 4): Preparation of2-(6,8-dichloro-2-(4-(trimethylstannyl)phenyl)-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide

2-(2-(4-bromophenyl)-6,8-dichloro-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide(0.5 g, 1.04 mmol) was dissolved in (dioxane, 20 ml), and thenhexamethylditin (0.43 mL, 2.08 mmol) andtetrakis(triphenylphosphin)palladium (0.17 g, 0.15 mmol) were addedthereto. The mixture was stirred at 120° C. under an argon atmospherefor 6 hours. After the temperature was lowered, the reaction mixture wasfiltered through celite, and then extracted with water and ethylacetate. The extracted organic layer was dried over sodium sulfate, andthe remaining solvent was removed under reduced pressure. The residuewas purified by column chromatography to afford the desired compound.

¹H NMR (400 MHz, CDCl₃) δ 0.32 (s, 3H, CH₃Sn), 0.67 (t, J=7.4 Hz, 3H,CH₃), 0.86 (t, J=7.4 Hz, 3H, _(CH3))_(,) 1.4-1.6 (m, 4H, CH₂), 3.06 (t,J=7.7 Hz, 2H, CH₂NCO), 3.29 (t, J=7.7 Hz, 2H, CH₂NCO), 4.09 (s, 2H,CH₂CO), 7.30 (d, J=1.9 Hz, 1H, Ar), 7.59 (d, J=7.8 Hz, 2H, Ar), 7.63 (d,J=7.8 Hz, 2H, Ar), 8.36 (d, J=1.9 Hz, 1H, Ar); ¹³C NMR (100 MHz, CDCl₃)δ 167.3, 145.7, 143.0, 141.3, 136.2, 133.6, 128.5, 124.7, 123.4, 122.0,120.0, 117.4, 50.1, 48.2, 30.5, 22.4, 21.1, 11.4, 11.0, −9.4; Anal.Calculated for (C₂₄H₃₁Cl₂N₃OSn): C, 50.83; H, 5.51; N, 7.41%. Found: C,51.08; H, 5.52; N, 7.43%. ESI-MS: calculated for [M+Na]⁺=590. Found=590:IR (KBr) : 1645 cm⁻¹.

According to the method for producing a fluorine-18-labeled PETradiotracer for targeting translocator protein overexpression accordingto the present invention, a product having high molar activity can beobtained. According to the method, in order to make fluorine-18 easy tohandle in mass production, a fluorine-18 reaction solution is preparedby adding, to CH₃CN, water containing fluorine-18 dissolved therein. Inorder to use a diaryliodonium salt whose aromatic ring compound can belabeled with nucleophilic fluorine-18 without having to use a separateelectron-attracting functional group,2-(6,8-dichloro-2-(4-(trimethylstannyl)phenyl)-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamideis produced and reacted with 4-(diacetoxy)iodoarene to obtain a((4-(6,8-dichloro-3-(2-(dipropylamino)-2-oxoethyl)imidazo[1,2-a]pyridin-2-yl)phenyl)(aryl)iodonium) anion precursor. This iodonium salt precursor isdissolved in CH₃CN to obtain an iodonium salt reaction solution. Byreacting the fluorine-18 reaction solution with the iodonium saltreaction solution, a radiotracer composition ([¹⁸F]BS224) containing afluorine-18-labeled radiotracer compound may be easily obtained.

In the present invention, the step of producing the((4-(6,8-dichloro-3-(2-(dipropylamino)-2-oxoethyl)imidazo[1,2-a]pyridin-2-yl)phenyl)(aryl)iodonium) anion precursor by reacting2-(6,8-dichloro-2-(4-(trimethylstannyl)phenyl)-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamidewith 4-(diacetoxy)iodotoluene may be performed, as shown in ReactionScheme 5 below.

Meanwhile, the step of preparing the fluorine-18 reaction solution maybe performed by further adding cesium hydrogen carbonate/18-crown-6 toCH₃CN to promote the reactivity of fluorine.

The production method may further include the first purification step ofpurifying the radiotracer composition by adding an aqueous hydrochloricacid solution to the radiotracer composition, followed by adsorptiononto a C18 Sep-Pak cartridge, washing with water, and then elution withethanol. In addition, the production method may further include thesecond purification step of purifying the radiotracer composition usinga high-performance liquid chromatography (HPLC) system equipped with a244 to 264 nm UV detector and a radioisotope gamma-ray detector. Throughthese purification steps, it may be possible to produce afluorine-18-labeled PET radiotracer ([¹⁸F]BS224) for targetingtranslocator protein overexpression having higher purity and activity.

To use the ability of the 2-aryl-6,8-dichloroimidazopyridine derivativeto be specifically taken up into an inflammation or tumor region, a PEGchain-substituted 2-aryl-6,8-dichloroimidazopyridine derivative having asensitizer introduced thereto as shown in Formula 3 above may beproduced by introducing, for example, an IR-780 compound, through amidebonding, as shown in Reaction Scheme 6 below:

This compound produced by introducing the photosensitizer IR-780compound to the 2-aryl-6,8-dichloroimidazopyridine derivative has thefollowing advantages when used in photodynamic therapy. Since IR-780acts as a compound that generates heat in cells, into which it has beentaken up, when receiving light having a specific wavelength (780 to 800nm), a therapy that burns a tumor by heat generated by irradiating lightinto a region into which the compound has been taken up may be appliedeven to a case in which surgery for selectively removing a tumor such asa bladder tumor is difficult and to a tumor therapy in which thefunctionality of an organ is highly likely to be lost due to surgery.

The present invention also provides a fluorine-18-labeled PETradiotracer ([¹⁸F]BS224) for targeting translocator proteinoverexpression represented by Formula 5 below, which is produced by amethod including the steps of: preparing a water-evaporated fluorine-18reaction solution by adding water containing fluorine-18 dissolvedthereto to CH₃CN, followed by heating to a temperature of 85 to 95° C.;producing a((4-(6,8-dichloro-3-(2-(dipropylamino)-2-oxoethyl)imidazo[1,2-a]pyridin-2-yl)phenyl)(aryl)iodonium) anion precursor as an iodonium salt precursor byreacting a 2-trimethyltinaryl-6, 8-dichloroimidazopyridine derivativewith 4-(diacetoxy)iodoarene; preparing an iodonium salt precursorreaction solution by dissolving the precursor and2,2,6,6-tetramethyl-1-piperidinyloxy in CH₃CN; and producing aradiotracer composition containing a fluorine-18-labeled radiotracercompound by adding the iodonium salt precursor reaction solution to thefluorine-18 reaction solution, followed by heating and reaction.

That is, the fluorine-18-labeled PET radiotracer ([¹⁸F]BS224,2-(6,8-dichloro-2-(4-[¹⁸F]fluorophenyl)imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide)for targeting translocator protein overexpression according to thepresent invention makes it possible to obtain new images ofneuroinflammation and tumors associated with translocator proteinoverexpression by positron emission tomography, thus diagnosing patientswith various brain diseases and tumors associated with translocatorprotein overexpression. In addition, by virtue of the long half-life offluorine 18, the PET radiotracer of the present invention can providebrain neuroinflammation and tumor imaging diagnostics to a larger numberof patients compared to conventional carbon-11 tracers.

In the present invention, a 2-pyridyl-6,8-dichloroimidazopyridinederivative may be synthesized as shown in Reaction Scheme 6 such that ithas a pyridine ring instead of the benzene ring on the right side of the[¹⁸F]BS224 compound. The release of the synthesized derivative compoundin a normal brain is suppressed, and thus the synthesized derivativecompound may more rapidly provide the difference between aTSPO-overexpressing region and normal cells.

Example 1: Synthesis of 2-(6,8-dichloro-2-(4-[¹⁸F]fluorophenyl)imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide usingiodonium salt precursor

50 to 100 μL of water having dissolved therein fluorine-18 produced froma cyclotron was added to a solution of CsHCO₃ (1.0 mg) and 18-crown-6(10.0 mg) in acetonitrile (container 1). The solvent was completelyevaporated by azeotropic distillation while the solution was heated to90° C. under nitrogen. An iodonium salt precursor (4 mg,(4-(6,8-dichloro-3-(2-(dipropylamino)-2-oxoethyl)imidazo[1,2-a]pyridin-2-yl)phenyl)(p-tolyl)iodonium) and 1 mg of 2,2,6,6-tetramethyl-1-piperidinyloxy weredissolved in 0.3 ml of acetonitrile (container 2), and then the solutionwas added to container 1, followed by reaction by heating at 140° C. for10 minutes. After the reaction, 10 mL of a 0.1 N aqueous solution of HClwas added to the reaction product which was then adsorbed onto aC18-light Sep-Pak cartridge, washed with 10 mL of water, and then elutedwith 0.5 mL of ethanol. The eluted solution was purified using an HPLCsystem (Waters, Xterra Semi-preparative C18 column, 10×250 mm, 10 μm;50% acetonitrile-water, 254 nm, flow rate: 5.0 mL/min) equipped with a254 nm UV detector and a radioisotope gamma-ray detector, andfluorine-18-labeled [¹⁸F]BS224 was isolated with a radiochemical yieldof about 25% at 34 minutes.

Example 2: Synthesis of 2-(6,8-duchloro-2-(4-[¹⁸1]fluorophenyl)imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide usingboron ester precursor

50 to 100 μL of water having dissolved therein fluorine-18 produced froma cyclotron was added to a solution of CsHCO₃ (1.1 mg) and 18-crown-6(3.5 mg) or K₂CO₃ (0.8 mg) and K₂₂₂ (5.0 mg) in acetonitrile (container1). The solvent was completely evaporated by azeotropic distillationwhile the solution was heated to 90° C. under nitrogen. A boron esterprecursor (3 mg,2-(6,8-dichloro-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)imidazo[1,2-a]pyridin-3-yl-N,N-dipropylacetamide)and a copper catalyst (0.2 mg of copper(II) trifluoromethanesulfonate,or 0.4 mg of tetrakis(pyridine)copper(II) triflate) were dissolved in0.5 ml of acetonitrile (container 2), and then the solution was added tocontainer 1, followed by reaction by heating at 110° C. for 10 minutes.After the reaction, 10 mL of a 0.1 N aqueous solution of HCl was addedto the reaction product which was then adsorbed onto a C18-light Sep-Pakcartridge, washed with 10 mL of water, and then eluted with 0.5 mL ofethanol. The eluted solution was purified using an HPLC system (Waters,Xterra Semi-preparative C18 column, 10×250 mm, 10 μm; 50%acetonitrile-water, 254 nm, flow rate: 5.0 mL/min) equipped with a 254nm UV detector and a radioisotope gamma-ray detector, andfluorine-18-labeled [¹⁸F]BS224pyridine was isolated with a radiochemicalyield of about 9% at 34 minutes.

Example 3: Production of2-(6,8-dichloro-2-(4-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide

To a solution of 18-crown-6 (25.0 μmol) in acetonitrile (0.3 mL), asolution of cesium fluoride (25.0 μmol) in water (10 μL) was added. Thesolvent was completely evaporated by azeotropic distillation while thesolution was heated at 90° C. under nitrogen. To the remaining material,a solution of(4-(6,8-dichloro-3-(2-(dipropylamino)-2-oxoethyl)imidazo[1,2-a]pyridin-2-yl)phenyl)(p-tolyl)iodonium tosylate in acetonitrile (0.3 ml) was added, followedby heating at 80 to 140° C. for 1 hour with stirring. After cooling toroom temperature, the reaction mixture was purified using an HPLC system(Waters, Xterra Semi-preparative C18 column, 10×250 mm, 10 μm; 50%acetonitrile-water, 254 nm, flow rate: 5.0 mL/min) equipped with a 254nm UV detector and a radioisotope gamma-ray detector, and2-(6,8-dichloro-2-(4-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide(BS224) was isolated at about 34 minutes.

¹H NMR (400 MHz; CDCl₃) δ 8.26 (d, J=1.6 Hz, 1H) , 7.63 (dd, J=5.6 Hz,8.8 Hz, 2H), 7.29 (d, J=2.0 Hz, 1H), 7.15 (t, J=8.8 Hz, 2H), 4.04 (s,2H), 3.29 (t, J=7.6 Hz, 2H), 3.10 (t, J=7.6 Hz, 2H), 1.44-1.60 (m, 4H),0.86 (t, J=7.2 Hz, 3H), 0.75 (t, J=7.2 Hz, 3H); ¹³C NMR (100 MHz; CDCl₃)δ 167.2, 164.3, 161.8, 144.7, 141.2, 130.9, 130.8, 130.7, 130.6, 130.0,129.9, 124.9, 124.7, 123.4, 121.8, 121.7, 120.0, 117.3, 116.1, 115.9,115.8, 50.0, 48.2, 30.3, 22.4, 21.0, 11.5, 11.4, 11.2, 11.1; I⁹F NMR(375 MHz; CDCl₃) δ −113.2; Anal. Calculated for (C₂₁H₂₂Cl₂FN₃O) : C,59.72; H, 5.25; N, 9.95%. Found: C, 59.77; H, 5.21; N, 9.98%. ESI-MS:calculated for [M−H]⁻=421. Found=421: IR (KBr): 1640 cm⁻¹.

Example 4: Production of2-(6,8-dichloro-2-(4-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide

(Step 1) Production of 4-(4-fluorophenyl)-4-oxo-N,N-dipropylbutanamide

3-(4-fluorobenzoyl)propionic acid (5.0 g, 25.5 mmol) and1,1′-carbonyldiimidazole (4.5 g, 28.0 mmol) were dissolved intetrahydrofuran (THF, 50 ml) and stirred at room temperature for 30minutes. After 30 minutes, N,N′-dipropylamine (4.2 mL, 30.6 mmol) andTEA (4.6 mL, 33.1 mmol) were added to the reaction solution which wasthen stirred for 4 hours. After the solvent was removed under reducedpressure, the residue was treated with a 0.1N aqueous solution ofhydrochloric acid, and then extracted with ethyl acetate. The extractedorganic layer was dried over sodium and then filtered, and the organicsolvent was removed under reduced pressure. The residue was purified bycolumn chromatography to afford the desired compound.

¹H NMR (400 MHz, CDCl₃) d 0.86 (t, J=7.4 Hz, 3H, CH₃), 0.95 (t, J=7.4Hz, 3H, CH₃), 1.53 (q, J=7.4 Hz, 2H, CH₂), 1.65 (q, J=7.4 Hz, 2H, CH₂),2.77 (t, J=7.4 Hz, 2H, CH₂CO), 3.2-3.4 (m, 6H, CH₂N+CH₂CO), 7.12 (d,J=6.8 Hz, 2H, Ar), 7.90 (d, J=6.8 Hz, 2H, Ar); Anal. Calculated for(C16H22FNO₂) : C, 68.79; H, 7.94; N, 5.01%. Found: C, 68.94; H, 7.90; N,5.03%. MS: calculated for [M]⁺=279 Found: MS m/z (% relative to the basepeak)=279 (5, M+), 179 (base). IR (KBr): 1640, 1685 cm⁻¹.

(Step 2) Production of3-bromo-4-(4-fluorophenyl)-4-oxo-N,N-dipropylbutanamide

To a solution of 4-(4-fluorophenyl)-4-oxo-N,N-dipropylbutanamide in atetrachlorocarbon solvent, a solution of Br2 (0.6 mL, 11.8 mmol) in atetrachlorocarbon solvent was added slowly. The reaction mixture wasstirred for 2 hours, and then the solvent was removed under reducedpressure. The residue was treated with a saturated aqueous solution ofsodium hydrogen carbonate, and then extracted with ethyl acetate. Theextracted organic layer was evaporated under reduced pressure, and theresidue was purified by column chromatography to afford the desiredcompound.

¹H NMR (400 MHz, CDCl₃) d 0.83 (t, J=7.4 Hz, 3H, CH₃), 0.98 (t, J=7.4Hz, 3H, CH₃), 1.49 (q, J=7.4 Hz, 2H, CH₂), 1.66 (q, J=7.4 Hz, 2H, CH₂),3.02 (dd, J,=16.0 Hz, J_(2=4.4) Hz, 1H, CH₂CO), 3.1-3.4 (m, 4H, CH₂N),3.57 (dd, J,=16.0 Hz, J_(3=9.9) Hz, 1H, CH₂CO), 5.59 (dd, J_(3=9.9) Hz,J_(2=4.4) Hz, 1H, CHBr), 7.12 (d, J=6.8 Hz, 2H, Ar), 7.90 (d, J=6.8 Hz,2H, Ar); Anal. Calculated for (C₁₆H₂₁BrFNO₂) : C, 53.64; H, 5.91; N,3.91%. Found: C, 53.73; H, 5.89; N, 3.93%. MS: calculated for [M]⁺=358Found: MS m/z (% relative to the base peak)=358 (0.9, M+) , 259 (base);IR (KBr) : 1690, 1687 cm⁻¹.

(Step 3) Production of2-(6,8-dichloro-2-(4-fluorophenyl)imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide

To a solution of 3-bromo-4-(4-fluorophenyl)-4-oxo-N,N-dipropylbutanamide(2.0 g, 5.6 mmol) in dimethylformamide (DMF, 25 mL),2-amino-3,5-dichloropyridine (1.2 g, 7.3 mmol) was added. The reactionmixture was stirred under reflux for 12 hours. After 12 hours, thesolvent was removed under reduced pressure, and the residue wasdissolved in ethyl acetate and extracted by treatment with a 0.1Naqueous solution of hydrochloric acid. The extracted organic layer wasevaporated under reduced pressure and then dried over sodium sulfate,and the solvent was removed under reduced pressure. The residue waspurified by column chromatography to afford the desired compound.

¹H NMR (400 MHz, CDCl₃) d 0.77 (t, J=7.4 Hz, 3H, CH₃), 0.86 (t, J=7.4Hz, 3H, CH₃), 1.2-1.3 (m, 4H, CH₂), 3.12 (t, J=7.7 Hz, 2H, CH₂NCO), 3.30(t, J=7.7 Hz, 2H, CH₂NCO), 4.04 (s, 2H, CH₂CO), 7.30 (d, J=1.9 Hz, 1H,Ar), 7.23 (d, J=8.2 Hz, 2H, Ar), 7.56 (d, J=8.2 Hz, 2H, Ar), 8.23 (d,J=1.9 Hz, 1H, Ar); Anal. Calculated for (C₂₁H₂₂Cl₂FN₃O): C, 59.72; H,5.25; N, 9.95%. Found: C, 59.77; H, 5.21; N, 9.98%. ESI-MS: calculatedfor [M−H]⁻=421. Found=421. IR (KBr) : 1640 cm⁻¹.

Example 5: Production of(4-(6,8-dichloro-3-(2-(dipropylamino)-2-oxoethyl)imidazo[1,2-a]pyridin-2-yl)phenyl)(p-tolyl)iodoniumtosylate

4-(diacetoxy)iodotoluene (235.2 mg, 0.7 mmol) was dissolved inacetonitrile (4 mL), para-toluene sulfonic acid (133.2 mg, 0.7 mmol) wasadded thereto, and then immediately the reaction solution was diluted inchloroform (20 ml). After the reaction was stirred at room temperaturefor 5 minutes, a solution of2-(6,8-dichloro-2-(4-(trimethylstannyl)phenyl)-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylamidein chloroform (4 mL) was added slowly thereto. The reaction mixture wasstirred at 50° C. for 18 hours. After cooling to room temperature, theorganic solvent was removed under reduced pressure. The residue wasextracted with dichloromethane and water, and the extracted organiclayer was evaporated under reduced pressure. The obtained oil wasdissolved in a small amount of dichloromethane and diethyl ether(v/v=1:1), and then added slowly to a conical tube containing colddiethyl ether (10 mL). Next, the solid was separated by centrifugation.

¹H NMR (400 MHz; CDCl₃) δ ppm: 8.03 (d, J=1.6 Hz, 1H), 7.99 (d, J=8.4Hz, 2H), 7.77 (d, J=8.4 Hz, 2H), 7.64 (d, J=8.4 Hz, 2H), 7.53 (bd, J=7.6Hz, 2H), 7.29 (d, J=1.6 Hz, 1H), 7.11 (d, J=8.0 Hz, 2H), 7.03 (bd, J=7.6Hz, 2H), 4.02 (s, 2H), 3.32-3.26 (m, 2H), 3.23-3.17 (m, 2H), 2.34 (s,3H), 2.29 (s, 3H), 1.60-1.50 (m, 4H), 0.86 (t, J=7.6 Hz, 3H), 0.80 (t,J=7.2 Hz, 3H); ¹³CNMR (100 MHz; CDCl₃) δ ppm: 166.8, 143.3, 142.9,141.3, 139.6, 137.4, 135.5, 135.1, 132.7, 131.8, 128.7, 126.1, 125.3,123.7, 121.5, 120.1, 118.7, 115.0, 111.9, 50.1, 48.2, 29.8, 22.4, 21.5,21.4, 21.1, 11.5, 11.3; MS (ESI) m/z 620 (M⁺-OTs) . HRMS calcd. forC₂₈H₂₉Cl₂IN₃O: 620.0732, found: 620.0735.

Example 6: Production of2-(6,8-dichloro-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)imidazo[1,2-a]pyridin-3-yl-N,N-diporpylacetamide

To a solution of2-(2-(4-bromophenyl)-6,8-dichloro-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide(230 mg, 0.48 mmol) in dimethylformamide (DMF, 15 mL),[1,1′-bis(diphenylphosphino)ferrocene]palladium(II) dichloride (41 mg,0.05 mmol), potassium acetate (141 mg, 1.4 mmol) andbis(pinacolato)diboron (134 mg, 0.53 mmol) were added. The reactionmixture was stirred at 80° C. for 3 hours. After the temperature waslowered to room temperature, the reaction mixture was filtered throughcelite, and then extracted with water and dichloromethane. The extractedorganic solvent was dried over sodium sulfate, and then the solvent wasremoved under reduced pressure. The residue was purified by columnchromatography to afford the desired compound.

¹H NMR (400 MHz, CDCl₃) δ 0.67 (t, J=7.4 Hz, 3H, CH₃), 1.37 (s, 12H,CH₃), 0.86 (t, J=7.4 Hz, 3H, CH₃), 1.4-1.6 (m, 4H, CH₂), 3.09 (t, J=7.7Hz, 2H, CH₂NCO), 3.29 (t, J=7.7 Hz, 2H, CH₂NCO), 4.07 (s,2H, CH₂CO),7.29 (d, J=2.0 Hz, 1H, Ar), 7.67 (d, J=8.0 Hz, 2H,

Ar), 7.89 (d, J=8.0 Hz, 2H, Ar), 8.28 (d, J=1.9 Hz, 1H, Ar); NMR (100MHz, CDCl₃) δ 167.2, 145.2, 141.2, 136.2, 135.1, 128.1, 124.6, 123.3,121.7, 119.4, 117.6, 83.9, 49.9, 48.0, 31.6, 30.2, 24.9, 22.4, 22.2,20.9, 14.1, 11.3, 11.0. ESI-MS: calculated for [M+H]⁺=530. Found=530:

Experimental Example 1: Measurement of In Vitro Stability in Human Serum

In an experiment on the in vitro stability of the fluorine-18-labeledPET radiotracer ([¹⁸F]BS224) for targeting translocator proteinoverexpression, 0.4 mL of human serum was mixed with 0.4 mL of 1%ethanol/water containing the fluorine-18-labeled radiotracer([¹⁸F]BS224), and then the solution was incubated at 37° C. for 0, 10,30, 60 and 120 minutes and then analyzed by thin film chromatography. Asa result of the analysis, it was confirmed that the fluorine-18-labeledradiotracer ([¹⁸F] BS224) was at least 98.7% stable for up to 120minutes, indicating that the fluorine-18-labeled radiotracer ([¹⁸F]BS224) had sufficient stability to perform in vivo biological studies.

Experimental Example 2: Preparation of LPS-Induced Neuroinflammatory RatModel

For the preparation of a neuroinflammatory rat model, maleSprague-Dawley rats weighing 200 to 250 g were used. The rats wereanesthetized, the skull was exposed, and a small hole was puncturedusing a bone drill. Next, 50 μg of LPS was infused into thepredetermined right striatum by the use of a Hamilton syringe at a flowrate of 0.5 μL/min (AP, 0.8 mm; L, −2.7 mm and P, −5.0 mm from thebregma). The Hamilton syringe was sustained in place for 10 min to avoidthe backflow of LPS. The small hole in the skull was filled with wax,and the incised scalp was sutured.

Experimental Example 3: Middle Artery Cerebral artery Occlusion (MCAO)Rat Model

7-Week-old Sprague-Dawley rats were prepared and acclimated for 1 week,and then about 8-week-old rats (weighing 300 g) were subjected torespiratory anesthesia with isoflurane to prepare a middle cerebralartery occlusion model. The rats were laid on their side, and then thefeet and head were fixed. Next, the hair on the neck was removed, andthe skin was incised about 3 cm. After removing the fat layer under theepidermis, the left carotid artery was exposed to secure the surgicalspace. After blocking the blood flow by tying the lower part of thecommon carotid artery, the blood flow was blocked by tying both sides atintervals sufficient to insert a probe into the internal carotid artery.After a small incision was made in the blood flow-blocked portion of theinternal carotid artery, a nylon probe was inserted through the incisionto the middle cerebral artery. After inserting the probe, the incisedneck portion was closed with a stapler, and then the blood flow wasblocked for 60 minutes. After removing the stapler, the probe wascarefully pulled out and the threads on both sides of the internalcarotid artery were removed. Among the two threads, the thread in thedirection of the middle cerebral artery was tied again, and then thethread in the total carotid artery was removed. The incised neck portionwas sutured.

Experimental Example 4: Measurement of Lipophilicity

For the measurement of lipophilicity, [¹⁸F]BS224 dispersed in 5%ethanol/saline was added to and mixed with n-octanol (5 mL) and sodiumphosphate buffer (0.15 M, pH 7.4, 5.0 mL), and then lipophilicity wasmeasured five times. Samples of each phase were counted forradioactivity, and the lipophilicity was expressed as the ratio of thecounts per minute from n-octanol versus that of the sodium phosphatebuffer. The lipophilicity of [¹⁸F]BS224 was 2.78±0.4.

Experimental Example 5: Measurement of In Vitro Binding Affinity

The measurement of the in vitro binding affinities of BS224 for TSPO andCBR was performed by substitution reactions with [³H]PK11195 and[³H]flunitrazepam [see Callaghan, P. D. et al. Comparison of in vivobinding properties of the 18-kDa translocator protein (TSPO) ligands[¹⁸F]PBR102 and [¹⁸F]PBR111 in a model of excitotoxin-inducedneuroinflammation. Eur. Nucl. Med. Mol. Imaging 42, 138-51 (2015)].

While the present invention has been described in conjunction with theexemplary embodiments and the drawings, the present invention is notlimited to the above-described embodiments, but it will be apparent tothose having ordinary knowledge in the art to which the presentinvention pertains that various modifications and alterations may bemade based on the foregoing description.

Therefore, the technical spirit of the present invention should not bedefined based on only the above-described embodiments, but should bedefined based on the claims as well as equivalents thereto.

INDUSTRIAL APPLICABILITY

The present invention provides a method for producing afluorine-18-labeled PET radiotracer for targeting translocator proteinoverexpression, the method including the steps of: preparing asolvent-evaporated fluorine-18 reaction solution by adding water havingfluorine-18 dissolved therein to CH₃CN, followed by heating to atemperature of 85 to 95° C.; producing a((4-(6,8-dichloro-3-(2-(dipropylamino)-2-oxoethyl)imidazo[1,2-a]pyridin-2-yl)phenyl)(aryl)iodonium) anion precursor as an iodonium salt precursor byreacting2-(6,8-dichloro-2-(4-(trimethylstannyl)phenyl)-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamidewith 4-(diacetoxy)iodotoluene; producing2-(6,8-dichloro-2-(4-(4,4,5,6-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)imidazo[1,2-a]pyridin-3-yl-N,N-dipropylacetamideas a boron ester precursor by reacting2-(2-(4-bromophenyl)-6,8-dichloro-imidazo[1,2-a]pyridin-3-yl)-N,Ar-dipropylacetamidewith bis(pinacolato)diboron; preparing an iodonium salt reactionsolution by dissolving the iodonium salt precursor and2,2,6,6-tetramethyl-1-piperidinyloxy in acetonitrile; preparing a boronester precursor reaction solution by dissolving the boron esterprecursor and a copper catalyst (copper(II) trifluoromethanesulfonate,or tetrakis(pyridine)copper(II) triflate) in dimethylformamide; andproducing a radiotracer composition containing a fluorine-18-labeledradiotracer compound by adding the iodonium salt or boron esterprecursor reaction solution to the fluorine-18 reaction solution,followed by heating and reaction.

The use of the fluorine-18-labeled PET radiotracer for targetingtranslocator protein radiotracer according to the present inventionmakes it possible to diagnose patients with various brain diseases andtumors by obtaining new images of neuroinflammation, stroke and tumorsassociated with translocator protein overexpression through PET. Inaddition, by virtue of the long half-life of fluorine 18, the PETradiotracer of the present invention can provide brain neuroinflammationand tumor imaging diagnostics to a larger number of patients compared toconventional carbon-11 tracers.

What is claimed is:
 1. A method for producing a fluorine-18-labeledpositron emission tomography (PET) radiotracer for targetingtranslocator protein overexpression, the method comprising the steps of:preparing a solvent-evaporated fluorine-18 reaction solution by addingwater having fluorine-18 dissolved therein to acetonitrile, followed byheating to a temperature of 85 to 95° C.; producing an iodonium saltprecursor by reacting a 2-aryl-6,8-dichloroimidazopyridine derivativewith a (diacetoxy)iodoarene derivative; preparing an iodonium saltprecursor reaction solution by dissolving the iodonium salt precursorand 2,2,6,6-tetramethyl-1-piperidinyloxyl in acetonitrile; and producinga radiotracer composition containing a fluorine-18-labeled radiotracercompound by adding the iodonium salt precursor reaction solution to thefluorine-18 reaction solution, followed by heating and reaction.
 2. Themethod of claim 1, further comprising a first purification step ofpurifying the radiotracer composition by adding an aqueous hydrochloricacid solution to the radiotracer composition, followed by adsorptiononto a C18 Sep-Pak cartridge, washing with water, and then elution withethanol.
 3. The method of claim 1, further comprising a secondpurification step of purifying the composition using a high-performanceliquid chromatography (HPLC) system equipped with a 244 to 264 nm UVdetector and a radioisotope gamma-ray detector.
 4. The method of claim1, wherein the step of preparing the fluorine-18 reaction solution isperformed by further adding cesium hydrogen carbonate (CsHCO3) and18-crown-6 ([C₂H₄O]₆) to the acetonitrile.
 5. A method for producing afluorine-18-labeled positron emission tomography (PET) radiotracer fortargeting translocator protein overexpression, the method comprising thesteps of: preparing a solvent-evaporated fluorine-18 reaction solutionby adding water having fluorine-18 dissolved therein to acetonitrile,followed by heating to a temperature of 85 to 95° C.; producing a boronester precursor by reacting a 2-aryl-6,8-dichloroimidazopyridinederivative with bis(pinacolato)diboron; preparing a boron esterprecursor reaction solution by dissolving the boron ester precursor anda copper catalyst (copper(II) trifluoromethanesulfonate, ortetrakis(pyridine)copper(II) triflate) in dimethylformamide (DMF); andproducing a radiotracer composition containing a fluorine-18-labeledradiotracer compound by adding the boron ester precursor reactionsolution to the fluorine-18 reaction solution, followed by heating andreaction.
 6. The method of claim 5, further comprising a firstpurification step of purifying the radiotracer composition by adding anaqueous hydrochloric acid solution to the radiotracer composition,followed by adsorption onto a C18 Sep-Pak cartridge, washing with water,and then elution with ethanol.
 7. The method of claim 5, furthercomprising a second purification step of purifying the composition usinga high-performance liquid chromatography (HPLC) system equipped with a244 to 264 nm UV detector and a radioisotope gamma-ray detector.
 8. Themethod of claim 5, wherein the step of preparing the fluorine-18reaction solution is performed by further adding cesium hydrogencarbonate (CsHCO₃) and 18-crown-6 ([C₂H₄O]₆) to the acetonitrile.
 9. Afluorine-18-labeled positron emission tomography (PET) radiotracer fortargeting translocator protein overexpression, which is represented byFormula 1 below and produced by a method comprising the steps of:preparing a solvent-evaporated fluorine-18 reaction solution by addingwater having fluorine-18 dissolved therein to acetonitrile, followed byheating to a temperature of 85 to 95° C.; producing an iodonium saltprecursor by reacting a 2-aryl-6,8-dichloroimidazopyridine derivativewith a (diacetoxy)iodoarene derivative; preparing an iodonium saltprecursor reaction solution by dissolving the iodonium salt precursorand 2,2,6,6-tetramethyl-1-piperidinyloxyl in acetonitrile; and producinga radiotracer composition containing a fluorine-18-labeled radiotracercompound by adding the iodonium salt precursor reaction solution to thefluorine-18 reaction solution, followed by heating and reaction:

wherein R is ¹⁸F or ¹⁹F, X is C or N, and Y is C or N.
 10. A precursorfor synthesizing the fluorine-18-labeled positron emission tomography(PET) radiotracer for targeting translocator protein overexpression ofclaim 9, wherein a 2-fluoroaryl-6,8-dichloroimidazopyridine derivativeof Formula 1 is a 2-aryl-6,8-dichloroimidazopyridine derivativerepresented by Formula 2 below:

wherein X is C or N, and Y is C or N.
 11. The precursor of claim 10,wherein Z in Formula 2 is a functional group selected from the groupconsisting of iodobenzene tosylate, iodotoluene tosylate,2-iodo-1,3,5-trimethylbenzene tosylate, 4-iodoanisole tosylate,3-iodoanisole tosylate, 2-iodothiophene tosylate, 3-iodothiophenetosylate, iodobenzene bromide, iodotoluene bromide,2-iodo-1,3,5-trimethylbenzene bromide, 4-iodoanizole bromide,3-iodoanisole bromide, 2-iodocyophene bromide, 3-iodothiophene bromide,iodobenzene iodide, iodotoluene iodide, 2-iodo-1,3,5-trimethylbenzeneiodide, 4-iodoanisole iodide, 3-iodoanisole iodide, 2-iodothiopheneiodide, 3-iodothiophene iodide, iodobenzene triflate, iodotoluenetriflate, 2-iodo-1,3,5-trimethylbenzene triflate, 4-iodoanisoletriflate, 3-iodoanisole triflate, 2-iodothiophene triflate,3-iodothiophene triflate, and pinacol boron ester.
 12. Afluorine-18-labeled positron emission tomography (PET) radiotracer fortargeting translocator protein overexpression, which is represented byFormula 1 below and produced by a method comprising the steps of:preparing a solvent-evaporated fluorine-18 reaction solution by addingwater having fluorine-18 dissolved therein to acetonitrile, followed byheating to a temperature of 85 to 95° C.; producing a boron esterprecursor by reacting a 2-aryl-6,8-dichloroimidazopyridine derivativewith bis(pinacolato)diboron; preparing a boron ester precursor reactionsolution by dissolving the boron ester precursor and a copper catalyst(copper(II) trifluoromethanesulfonate, or tetrakis(pyridine)copper(II)triflate) in dimethylformamide (DMF); and producing a radiotracercomposition containing a fluorine-18-labeled radiotracer compound byadding the boron ester precursor reaction solution to the fluorine-18reaction solution, followed by heating and reaction:

wherein R is ¹⁸F or ¹⁹F, X is C or N, and Y is C or N.
 13. A precursorfor synthesizing the fluorine-18-labeled positron emission tomography(PET) radiotracer for targeting translocator protein overexpression ofclaim 12, wherein a 2-fluoroaryl-6,8-dichloroimidazopyridine derivativeof Formula 1 is a 2-aryl-6,8-dichloroimidazopyridine derivativerepresented by Formula 2 below:

wherein X is C or N, and Y is C or N.
 14. The precursor of claim 13,wherein Z in Formula 2 is a functional group selected from the groupconsisting of iodobenzene tosylate, iodotoluene tosylate,2-iodo-1,3,5-trimethylbenzene tosylate, 4-iodoanisole tosylate,3-iodoanisole tosylate, 2-iodothiophene tosylate, 3-iodothiophenetosylate, iodobenzene bromide, iodotoluene bromide,2-iodo-1,3,5-trimethylbenzene bromide, 4-iodoanizole bromide,3-iodoanisole bromide, 2-iodocyophene bromide, 3-iodothiophene bromide,iodobenzene iodide, iodotoluene iodide, 2-iodo-1,3,5-trimethylbenzeneiodide, 4-iodoanisole iodide, 3-iodoanisole iodide, 2-iodothiopheneiodide, 3-iodothiophene iodide, iodobenzene triflate, iodotoluenetriflate, 2-iodo-1,3,5-trimethylbenzene triflate, 4-iodoanisoletriflate, 3-iodoanisole triflate, 2-iodothiophene triflate,3-iodothiophene triflate, and pinacol boron ester.
 15. A fluorescentdye-labeled or sensitizer-labeled, translocator proteinoverexpression-targeting ligand tracer for fluorescence imaging-guidedsurgery and photodynamic therapy, which is represented by Formula 3below and produced by a method comprising the step of producing asensitizer-labeled ligand as a fluorescent ligand by reacting aprecursor of a 2-aryl-6,8-dichloroimidazopyridine derivative with afluorescent dye or a photodynamic therapy sensitizer, which has afunctional group for complementary bonding to the precursor:

wherein X is C or N; Y is C or N; the number (n) of polyethylene glycol(PEG) chains is 1 to 10; the linker that connects the PEG to thefluorescent dye or the photodynamic therapy sensitizer is a compoundselected from the group consisting of ether, amide, ester, urea,urethane, thiourea, and disulfide; and the PEG is substituted at any oneof the 2-, 3- or 4-positions of the ring containing X and Y.
 16. Thefluorescent dye-labeled or sensitizer-labeled, translocator proteinoverexpression-targeting ligand tracer of claim 15, wherein the linkeris a linker selected from the group consisting of ether, amide, ester,urea, urethane, thiourea, and disulfide.
 17. The fluorescent dye-labeledor sensitizer-labeled, translocator protein overexpression-targetingligand tracer of claim 15, wherein the PEG chain-substituted2-aryl-6,8-dichloroimidazopyridine derivative having a fluorescent dyeor sensitizer introduced thereto as represented by Formula 3 above issynthesized from a PEG chain-substituted2-aryl-6,8-dichloroimidazopyridine derivative precursor of Formula 4below:

wherein X is C or N; Y is C or N, the number (n) of the polyethyleneglycol (PEG) chains is 1 to 10, and the PEG is substituted at any one ofthe 2-, 3- and 4-position of the ring containing X and Y.
 18. Thefluorescent dye-labeled or sensitizer-labeled, translocator proteinoverexpression-targeting ligand tracer of claim 15, wherein Z in Formula4 is a functional group selected from the group consisting of acid,alcohol, thiol, amine, isocyanate, isothiocyanate, bromide, iodide,chloride, N-succinimidyl ester, and sulfo-N-succinimidyl ester.